Cornwallis-West was born into an upper-class Irish family as the daughter of Reverend Frederick and Lady Olivia FitzPatrick, daughter of Thomas Taylour, 2nd Marquess of Headfort. Her mother unsuccessfully attempted to seduce Albert, Prince Consort, and Cornwallis-West herself became mistress of the Prince of Wales (later King Edward VII) at the age of 16. The affair was discovered and she was married to the twice older William Cornwallis-West in 1872.
Three children were born to Mary and William Cornwallis-West: a daughter named Daisy in 1873, a son named George the following year and finally Constance in 1878. Cornwallis-West became notorious for using her influence over the Prince of Wales to arrange marriages for her children, particularly the marriage of her younger daughter to the wealthy Duke of Westminster.
In 1915, she began a relationship with a much younger shell-shocked soldier, Patrick Barrett, nursed in the Duchess of Westminster's hospital in Le Touquet. Her attempts to have the soldier promoted within the army caused a scandal. She was widowed in 1917 and died three years later.